KiwiRail is ready to use mothballed locomotives to keep freight moving as fallout from the discovery of asbestos in its new freight haulers continues.

Forty new Chinese-built DL locomotives have been tested for asbestos since the toxic fibres were found inside one on Friday.

As a result, freight movement around the country has been hit. Eight old locomotives, replaced by the DLs last year, are being prepared to return to service.

The train workers' union has said it will not allow people to be put in danger by operating the Chinese trains until they are given the all-clear.

Testing of 33 of the engines has been completed, but no results have been released.

Rail and Maritime Transport Union general secretary Wayne Butson called on KiwiRail yesterday to maintain its quarantine of the trains until all asbestos had been cleared from the locomotives.

If it did not do this, the union - which has a membership rate of more than 90 per cent - would tell its members not to operate them, effectively meaning they could not run.

"It's going to make life very, very difficult for rail and I really feel sorry for the customers of KiwiRail," Butson said.

KiwiRail spokeswoman Jenny Austin said it would keep the Chinese trains off the run until they were safe to use.

Along with the eight old engines being readied, other locomotives had been moved across to the South Island to help replace the DLs.

Butson said numerous workers had spotted asbestos in the locomotives over the past few years but KiwiRail had told the union there was none.

The union would let its members back on the locomotives only if lab tests showed there was no asbestos present.

But given asbestos had long been suspected, and now found, this was extremely unlikely, Mr Butson said.

"If they have lied to us in the past, how do we know they are not lying to us now?"

Transport Minister Gerry Brownlee was questioned in Parliament yesterday by Labour transport spokeswoman Darien Fenton, and said he had confidence in KiwiRail management. He had sought more information about test results.

Ms Fenton asked him to confirm reports that two trains were so toxic they would need to be quarantined.